When Seals Molt, They Leave Behind Mercury

Pollution collects at the top of the food chain.

For decades, scientists have wondered why some parts of the California coastline experience annual spikes in the amount of mercury in the water. This week, a group of researchers have finally fingered a culprit: molting elephant seals.

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Mercury is a particular bugbear for environmental conservation because its most toxic form, a neurotoxin called “methyl mercury,” is easily absorbed by marine life. Despite attempts to prevent mercury from leaching into the environment, it still winds up in the ocean thanks to pollution and industrial runoff. But because mercury is an element, it doesn't break down easily. Instead, it sticks around, moving its way up the food chain until it is concentrated in the apex predators in a process known as “biomagnification.” And in this case, the mercury tends to stop at California’s seal and sea lion populations, David Wagner reports for KBPS Radio. It is then re-released into the ocean in several way, namely the seals’ annual “catastrophic molt,” according to Cossaboon’s study which was recently published in the Proceedings of the Royal Society B.

"Anywhere there are a lot of seals or sea lions that are eating and defecating and molting, I would say it would be something worth looking into," Cossaboon tells Wagner. "There's a good chance mercury would be elevated."

“I think it’s important when trying to understand the global mercury cycle, that we’re really looking at the whole picture,” Jennifer Cossaboon, a researcher at the University of California Santa Cruz, tells Chelsea Harvey for The Washington Post. “It’s really interesting to see how wildlife can be exacerbating the cycle.”

Seals flock to parts of the California coastline several times a year, namely to mate and to molt. The process is called a “catastrophic molt” because the animals shed their entire top layer of skin and all of their fun at the same time, leaving a mass of biological material floating in the water that just happens to contain large amounts of methyl mercury. Scientists have suspected this was the reason for annual spikes in mercury levels around areas like the seal rookery at Año Nuevo State Reserve since at least 1981, but instruments at the time weren’t sensitive enough to make a concrete determination about the source of the mercury. This time, Cossaboon and her team found that the waters around Año Nuevo had 17 times the mercury levels during seal molting season as waters without seals.

"At that time, we didn't have the analytical instruments to detect mercury at the concentrations found in seawater, so we used mussels, which filter seawater, as sentinel organisms," Russell Flegal, Cossaboon’s co-author and author of the 1981 study said in a statement. "In the new study, we were able to look at seasonal changes in the water, and during the elephant seal molting season the levels of methyl mercury really took off."

Because seals are at the top of the food chain, they are a “sentinel species” that can signal big problems in their environment. After all, seals don’t only gather at Año Nuevo, but can be found up and down California’s coastline. While the study points at the issue of mercury pollution in particular, it signals that pollutants may be affecting the environment in ways that scientists still don’t understand.

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About Danny Lewis

Danny Lewis is a multimedia journalist working in print, radio, and illustration. He focuses on stories with a health/science bent and has reported some of his favorite pieces from the prow of a canoe. Danny is based in Brooklyn, NY.